1. Field of the Invention:
The present invention relates to black toner for electrophotography used for an image forming apparatus employing so-called electrophotographic technology, especially, an electrostatic printing method, such as an electrostatic copying machine and a laser beam printer. More particularly, it relates to black toner used with color toners such as magenta, cyan, and yellow toners for full color image formation by the electrophotographic technology.
2. Description of the Prior Art:
In the field of electrophotographic technology, especially, in an electrostatic printing method, toner is used for visualizing an electrostatic latent image formed on an image-holding body. This toner is prepared by dispersing a colorant in a resin medium. As the resin medium, resin having a desired detecting property and a binding property, for example, various resins such as styrene resin and polyester resin are used. As the colorant, carbon black and other organic or inorganic color pigments are used.
There has been conducted full color development in recent years. The full color development is conducted in the following manner. First, a multicolor original is subjected to color separation treatment, after which each image is exposed to light. This process is repeated by the use of color toners of magenta, cyan, and yellow and black toner, respectively. The resulting images formed with each color toner are successively superimposed one over the other to form a multicolor image.
Various organic pigments are used as the colorant for each color toner mentioned above, and carbon black is used as the colorant for black toner.
The important factors which determine the characteristics of the carbon black are as follows:
(1) mean grain size; PA0 (2) structure; and PA0 (3) chemical properties of the grain surface.
The mean grain size is a factor which determines the colorability of carbon black, and measured by statistically evaluating an image observed through an electron microscope.
The structure is a term characteristic of carbon black and representing the degree to which carbon black particles are linked with one another. The structure can be relatively easily measured by evaluating the image observed through an electron microscope, and generally determined quantitatively by measuring the specific surface area of carbon black, and the oil absorption of carbon black when dibutyl phthalate is used as an oil.
For example, according to the data issued by Colombian Carbon Co., the conditions of carbon black particles are denoted in the following manner. The condition in which carbon black particles are linked with one another to a high degree is referred to as "high structure", the intermediate degree as "normal structure", and the low degree as "low structure". The degree of linkage is also represented by a structure index. The structure index of carbon black is determined in the following manner. First, the carbon black of normal structure is arbitrarily defined as "100". Then, the structure index of carbon black is determined based on a curve representing the relationship between the specific surface area and the oil absorption when dibutyl phthalate is used as an oil of the carbon black of normal structure (abscissa: specific surface, ordinate: oil absorption when dibutyl phthalate is used as an oil).
The chemical properties of the particle surface are generally determined by the kind and the content of an oxygen-containing component present on the particle surface, and ordinarily determined by the content of a volatile component.
Since conventional carbon black to be added in black toner has a high electrical conductivity unlike other colorants, and is used as a conductivity-imparting agent at is known, the electrical properties of black toner are largely different from those of other color toners.
Since colorants other than carbon black have only the same conductivity as a resin binder, the conductivity of color toner is substantially of the same order as a resin binder alone, while the conductivity of black toner containing carbon black is very much higher than that of the resin binder.
Accordingly, when both black toner and color toner having largely different electrical properties such as conductivity are used together to form images under the same conditions, there arises a difference in the amount of toner to be transferred and the like between the black toner and the color toner, making it impossible to reproduce vivid color images.
In order to overcome the above problem, the conditions for the formation of images may be changed in accordance with black toner and color toner. However, it is necessary to change the conditions whenever an image is formed. This causes the apparatus to operate unstably, thereby adversely affecting formed images, and involving a complicated structure for the apparatus.